Future spaceborne step and staring imaging systems will likely incorporate recent advances in both visible and infrared staring array technology. These advances include higher operating temperatures, improved signal to noise performance, lower power, larger pixel count (array format) and highly-integrated control and signal processing functions. For many applications, both higher-resolution panchromatic and lower-resolution multi-spectral imagery are desired. However, because of many considerations, including, for example, cost, size, weight, power, etc., separate panchromatic and multi-spectral optical sensors are generally not practical for spaceborne imaging systems.
In order to acquire optical imagery across different spectral bands using a single optical sensor, high-speed, mechanical filter-switching mechanisms have been proposed to move different spectral filters into, and out of, the optical path of a large format staring array. For example, a wide-band panchromatic filter may be moved into the optical path of the array and panchromatic data may be acquired after the optical signal has been integrated over a certain period of time. The panchromatic filter may then be moved out of the optical path, one or more narrow-band spectral filters may be sequentially moved into the optical path, the optical signal may be integrated and the narrow-band spectral data acquired. This multi-spectral data may be acquired at the same, or lower, resolution as the panchromatic data.
There are several disadvantages to this mechanically-switched approach, including, for example, the reliability of the mechanism, the disturbances imparted to the satellite due to the motion of the various components of the mechanism, as well as the size, weight, power, cost, etc., of the device. One of the most critical disadvantages of this approach is the time required to sequentially switch the spectral filters, integrate the optical signal, acquire and then transfer the higher-resolution data to the host satellite's telemetry system. These data include a large number multi-spectral pixels, which not only increase processing and data transfer loads, but also complicate pixel calibration. These critical disadvantages constrain the overall image gathering capability of the satellite, and may necessitate the deployment of additional satellites in order to provide sufficient coverage.
Thus, there is a need for a spectral filter that permits an imaging array to acquire both wide and narrow band imagery without the disadvantages attendant with a mechanical, filter-switching mechanism.